1. Field of the Invention
The present invention relates to a pervaporation method of separating a liquid organic compound mixture through an aromatic imide polymer asymetric membrane.
More particularly, the present invention relates to a pervaporation method of separating a liquid organic compound mixture through an aromatic imide polymer asymmetric membrane at a high selectivity and at a high permeation rate.
2. Description of the Related Art
It is known that a liquid mixture of two or more types of organic compounds can be separated into individual organic components by a distillation method, but in the distillation method, some of the organic compounds form an azeotropic mixture, or have boiling points close to each other, or are chemically modified at the distillation temperature, and therefore, it is very difficult to separate them from each other.
To avoid the above difficulty, a method of separating the liquid organic compound mixture by using a separating membrane has been attempted. In this method, wherein a membrane is used to separate or concentrate an organic compound aqueous solution, a diluted aqueous solution of organic compounds is brought into contact with a surface of a membrane, to allow a specific liquid component to selectively permeate through the membrane due to a differential osmotic pressure. This method is referred to as a reverse osmosis method.
Usually, in the reverse osmosis method for a aqueous solution, a higher pressure than the osmotic pressure of the aqueous solution must be applied to the aqueous solution, and therefore, the reverse osmosis method can not be applied to a concentrated organic compound aqueous solution which exhibits a high osmotic pressure, and accordingly, the reverse osmosis method can be applied only to organic compound aqueous solutions having a limited concentration.
Recently, as a new type of separating method in which a separating membrane is used, a pervaporation method has been developed for a liquid organic compound mixture and is now under serious consideration in this field.
In the pervaporation method, an organic compound mixture in the state of a liquid is brought into direct contact with a feed side face of a separating membrane capable of selectively allowing a specific organic compound to permeate therethrough, and the opposite delivery side face of the membrane is exposed to a vacuum or a reduced pressure. The specific compound is allowed to selectively permeate through the membrane and is collected in the state of a vapor at the opposite delivery side of the membrane. This method is useful for selectively separating or concentrating an individual organic compound from a liquid organic compound mixture.
Many proposed pervaporation methods have been reported. For example, Japanese Unexamined Patent Publication No. 52-111888 discloses a separation of a benzene-cyclohexane mixture solution or benzenehexane mixture solution by using an ionomer type polymer; and Japanese Unexamined Patent Publication No. 59-30441 discloses a separation of the above-mentioned mixture solution by using a polyamide membrane.
Nevertheless, the conventional separating membranes for the pervaporation method are disadvantageous in that;
(1) the permeation rate of the membrane for individual organic compounds to be separated or concentrated is unsatisfactorily low;
(2) the selectivity of the membrane for separating the individual organic compounds is unsatisfactory;
(3) the heat resistance and solvent resistance of the membrane are unsatisfactory; and
(4) the membrane has a low durability, and thus cannot be employed continuously over a long time; e.g., when separating various individual organic compounds from a mixture.
Also, Japanese Unexamined Patent Publication No. 63-7804 discloses, as a comparative example, a method of separating an acetic acid aqueous solution by pervaporation, in which a separating membrane is made of a pyromellitic acid type aromatic polyimide, and reports that this type of aromatic polyimide membrane exhibits a very poor separating property in the pervaporation method.
It is known that a separating membrane comprising a biphenyltetra-carboxylic acid-type aromatic polyimide can be used in the osmosis separating procedure for inorganic salt aqueous solutions but it is not known how to effectively carried out the pervaporation method by using an aromatic imide polymer separating membrane.